I have a
polyline, and i want to add
point (vertex) on a specific distance, like two meters for example.
How can i do that?
You should use the IPolyCurve:SplitAtDistance Method. This
Just remember to set the CreatePart to false. This way the polyline will not be split, but just a new vertex added.
There are existing tools to densify polylines. For example this link for ArcMap 10 however you indicate Arcobjects in your tag line, so this may not be appropriate for your situation.